Fast read-only transactions in replicated databases

The authors present a propagation mechanism, called the commit propagation mechanism (CPM), which increases the availability of data for read-only transactions. The proposed mechanism is piggy-backed on the messages used in the two-phase commit protocol. The CPM was combined with the standard quorum protocol in two different replicated database systems. In a fully replicated database, CPM allows any read-only transaction to execute locally at a single site without the need for any communication overhead. In a partially replicated database, CPM either ensures that the set of copies residing at a site are mutually consistent, or indicates which copies violate such consistency